commonly known as "if it exist, there is a porn of it" when in reality the rule is "if it will rape your childhood, there is a porn of it", therefore pr0nyou want will never exist, but there will always be Goof Troupe porn somewhere in the interwebs ether

From 7chan. Often users will take the miccy and send things gross, such as 100+ adult pornography, or some sicko would say "CHILDREN UNDER 10! RULE 34, GOOO" often with an awful lot of spam to show how stupid they actually are.

A rule stating that if a thing exists, there is pornography of that thing.

In other words, Proof that your childhood is not sacred.

"Rule 34: if it exists, there is porn of it"

Cases in point:

Super Hornio Bros
SpongeKnob SquareNuts
E3 The Extra Testicle
The Da Vinci Load
Forrest Hump
Hairy Squatter and the Sorcerer's Bone
Indiana Jones and the Temple of Poon
Eternal Cumshine of the Topless Mind (okay I just made that one up but you get the idea)